Artificial particulate systems such as polymeric beads and liposomes are finding a variety of biomedical applications in drug delivery, drug targeting, protein separation, enzyme immobilization and blood cell substitution (1-6). Liposomes have a flexible, cell-like lipid bilayer surface which acts as a permeability barrier such that compounds can be entrapped in their aqueous interior. However, liposomes can be mechanically unstable and their loading capacity limited by the water solubility of the material to be loaded (7). Polymeric beads, although mechanically more stable and having a larger loading capacity then liposomes, lack many of the useful surface properties of a lipid bilayer shell.
Lipid bilayers supported on various solid surfaces, such as glass (8), plastic (5), and metal (9) as well as modified polymers (10) have previously been shown to provide a stable and well defined cell membrane-like environment that has found a number of basic and applied uses (11,12). Gao and Huang reported that encapsulation of hydrogel particles into liposomes enhanced the loading capacity and overall mechanical strength of the liposomal structure (13). However, in that system the unanchored bilayer is still somewhat unstable and the system could only be formed with specific lipid mixtures and only with polymer cores of certain sizes and shapes. The inventors have developed a hydrogel anchored lipid vesicle in which these limitations have been overcome.